To cope with the auto-emission control which is made stricter with each passing year, there is recently used a catalyst-carrying honeycomb structure to eliminate nitrogen oxides, sulfur oxides, hydrogen chlorides, hydrocarbons, carbon monoxide, and the like contained in auto exhaust fumes. Further, the honeycomb structure is also used as a filter for capturing micro particles exhausted from diesel engines.
For example, since a catalyst used in the catalyst-carrying honeycomb structure is ordinarily more activated in a high temperature region, a vehicle is driven in the state in which the catalyst is less activated during the period from the start of the vehicle to the time at which the temperature of the honeycomb structure is increased, thereby insufficiently purified fumes are exhausted. Accordingly, the period of time during which the vehicle is driven at a low temperature of the honeycomb structure must be shortened as much as possible. One of methods for this purpose is to increase the temperature of the honeycomb structure in a short time from the start of the vehicle by reducing the heat capacity of the honeycomb structure. To reduce the heat capacity of the honeycomb structure, the weight (the density) of the honeycomb structure must be reduced without modifying the geometrical surface area thereof, and there is a method of reducing the thickness of partition walls and increasing a porosity for this purpose. However, the mechanical strength of the honeycomb structure is reduced by reducing the density thereof by reducing the wall thickness and increasing the porosity.
Further, a honeycomb structure for purifying a large flow amount of fumes exhausted from large vehicles such as trucks and the like must have a large volume, and further must have a large sectional area to reduce a pressure loss. However, a honeycomb structure having a large sectional area has a problem in that the partition walls of cells cannot endure the weight of the honeycomb structure itself and are deformed in the outer periphery of the honeycomb structure (refer to patent document 1).
To prevent the reduction of the mechanical strength, there is proposed a method of processing and eliminating the deformed portions of partition walls in the outer periphery of a honeycomb structure after it is formed and fired and forming an outer shell layer constituting the outer surface of the honeycomb structure by filling the recessed grooves of the outer periphery with a ceramic coating, and a honeycomb structure comprising the outer shell (refer to patent document 2). However, this method is disadvantageous in that the temperature increasing speed of the honeycomb structure is reduced at the start of operation because the heat capacity of the honeycomb structure is increased and thus it is difficult for the catalysts carried by the honeycomb structure to be activated in a short time, although the mechanical strength of the honeycomb structure can be enhanced. Further, there is also a problem in that the temperature of the central portion of the honeycomb structure is different from that of the outer periphery thereof. The occurrence of the temperature difference is not preferable because when the honeycomb structure is used as a catalyst carrier and a filter, catalysts are activated and the filter is recycled unevenly, respectively. Further, the occurrence of the temperature difference is also not preferable in that it may cause cracks to be generated in the honeycomb structure.
Further, there is a proposal for improving the accuracy of the outside diameter of a honeycomb structure which has partition walls and outer peripheral wall formed integrally by extrusion, by disposing a covering layer around the outer peripheral surface thereof (refer to patent document 3). The proposal discloses a method of suppressing the breakage of the honeycomb structure caused by the reduction of mechanical strength thereof in canning due to the reduced partition wall thickness by reducing canning surface pressure by properly setting the range of clearance in the canning by improving the accuracy of the outside diameter of the honeycomb structure. Although the means for forming the covering layer around the outer peripheral surface of the honeycomb structure, which has the partition walls and the outer peripheral wall made integrally, can improve the canning resistant property of the honeycomb structure, a problem arises in that the heat capacity of an outer wall part is increased and the heat in the honeycomb structure escapes to the outside wall thereof, the problem being the same as that occurring in the above-mentioned honeycomb structure whose outer periphery is coated with the ceramic cement.    Patent document 1: JP-A-3-275309    Patent document 2: JP-A-5-269388    Patent document 3: JP-U-63-144836